Updated pylint, pyyaml dependencies

[zenmonkeykstop]

Status

Ready for review

Description of Changes

updates dev/admin dependencies:

• pylint 2.5.0 -> 2.7.0 (python 3.6+ required, no review as dev only)
• pyyaml 5.3.1 -> 5.4.1 (python 3.6+ required, previously reviewed at https://github.com/freedomofpress/securedrop-debian-packaging/wiki/PyYAML-5.3.1-to-5.4.1 )
• updates CI lint job to use Focal instead of Xenial (because python 3.6+)

Testing

• CI is passing
• no new unreviewed dependencies

Deployment

• development dependencies pulled in with next manual pip install
• admin dependencies update with next GUI or manual update.

[kushaldas]

The change is good.

• CI is green
• no new dependencies.

CI checks moved to Focal based image. 👍🏽

Approved.